What is different from mainstream Protestants

Roman Catholics believe God forgives sins through the sacrament of reconciliation (penance), which is performed through a priest, while most Protestants do not believe in the sacrament.

Roman Catholics believe it is important to live by Scripture and Tradition, which the teaching of the Church's Magisterium (the bishops in communion with the Pope) come from, while most Protestants believe in Sola Scriptura (the Bible alone)

Roman Catholics believe that papal authority (in very specific, solemn occasions called "ex Cathedra") and the Bible are infallible, while most Protestants believe in an infallible Bible but not an infallible Pope. Papal infallibility has been declared twice in the history of the Catholic Church. Once to state that Mary was conceived without sin and another to state that Mary was assumed into heaven body and soul.

The Bible Roman Catholics use often include a few texts that are usually not used by Protestants. The best known are called the Deuterocanonical books.

Roman Catholics venerate saints, especially the Virgin Mary (Mother of God). "Venerating saints" means that Roman Catholics give special honor to saints (people in heaven) because they believe that saints can pray for them directly to God. Many Protestants do not, because they regard "venerating saints" as "worshiping saints". Because they believe that only God should be worshipped, they do not venerate. Many Protestants also simply do not believe that any veneration is necessary.